(1) Field of the Invention
The invention relates to a method of photolithographic etching of metal lines, and more particularly, to a method of photolithographic etching of sub-quarter micron metal lines without undercutting in the manufacture of integrated circuits.
(2) Description of the Prior Art
It is desired for metal lines to have a vertical profile. This is not always easy to achieve, especially for sub-quarter micron metal lines where the photoresist layer must become thinner in order to improve lithographic resolution. However, a thinner photoresist layer results in the etching away of the metal sidewall because of poor photoresist selectivity, especially with a deep ultraviolet (DUV) light source. Therefore, it is necessary to use a hard mask during metal etching in sub-quarter micron technology to protect the metal sidewalls.
FIG. 1 illustrates in cross-sectional representation a partially completed integrated circuit device of the prior art. Semiconductor substrate 10 contains semiconductor device structures, such as gate electrode 14 and source and drain regions 16. An opening has been made through the insulating layer 18 to one of the underlying source/drain regions 16. The via opening has been lined with a glue layer 22 and filled with a tungsten layer 26. A barrier metal layer 28 (for example, titanium/titanium nitride) covers the tungsten plug. A metal layer 36, such as AlCu, overlies the barrier layer. An anti-reflective coating (ARC) 38 covers the metal layer. FIG. 1 illustrates the integrated circuit device after the etching of the metal line. A portion of the hard mask 40 remains overlying the metal line stack. The conventional etchant gases BCl.sub.3 /Cl.sub.2 /N.sub.2 cannot provide an attack-free sidewall solution. The metal lines will be undercut 52 near the ARC interface as shown in FIG. 1. The N.sub.2 reacting with the photoresist forms a C--N chemical passivation layer. However, this layer cannot protect the metal sidewall completely from Cl.sub.2 erosion or B bombardment.
One proposed solution to the undercutting problem, disclosed in co-pending U.S. patent application Ser. No. 09/020,501 (TSMC-97-091) to Lee et al, is the use of SF.sub.6 gas in the overetch step to react with titanium from the barrier layer and AlCu to form AlF.sub.x or TiF.sub.x as a passivation layer. However, SF.sub.6 is the etching gas used in the tungsten etchback process. If tungsten plugs underlie the metal lines in the substrate, as illustrated in FIG. 1 and especially at the endcap of the metal line, as shown in FIG. 2, the SF.sub.6 etchant gas will damage the integrity of the tungsten plug 24. Also, if there is misalignment, the SF.sub.6 gas may react with Cl ions to create particles within the etch chamber.
Co-pending U.S. patent application Ser. No. 08/998,673 (TSMC-97-210) to Shue et al discloses the use of a fluorine-doped silicate glass hard mask as a source of fluorine ions for forming a passivation layer of AlF.sub.x or TiF.sub.x on the metal sidewalls to prevent undercutting. U.S. Pat. No. 5,219,485 to Wang et al teaches etching a polycide layer using BCl.sub.3 /Cl.sub.2 /HCl. U.S. Pat. No. 5,582,679 to Lianjin et al teaches using BCl.sub.3 /Cl.sub.2 /N.sub.2 at a high temperature to etch aluminum without undercutting. U.S. Pat. No. 5,578,163 to Yachi discloses the use of BCl.sub.3 and Cl.sub.2 to etch aluminum and then overetching with a compound containing at least a hydrogen atom to remove residual chlorine. In this patent, the sidewall is protected by a passivation layer generated by the photoresist.